Q2. What is the purpose of this letter?
In this Letter the application of femtosecond pulses to coherent Raman spectroscopy of molecular transitions in liquid systems is discussed.
Q3. What is the advantage of the system?
A major advantage of the system is the ready tunability of the excitation frequency wL- ws using the birefringent filter of thepicosecond laser.
Q4. Why are both pyridine modes excited in the experiment?
Due to the broad spectral widths of the femtosecond exciting force of Aw/27co 200 cm-’ both pyridine modes are simultaneously excited in the experiment.
Q5. How many peaks in the Fourier spectrum are there?
The authors found three peaks in the Fourier spectrum at 39,189, and 228 cm-‘, which correspond to the frequency differences between the three modes excited in their mixture.
Q6. What is the time delay between the exciting and probing pulses?
The coherent signal S, is recorded as a function of the delay time (set by the optical delay line VD) between the exciting and probing pulses EL and EL*, respectively.
Q7. How long is the unidirectional ring laser?
The length of the unidirectional ring laser is actively controlled in order to synchronize the pulses from the femtosecond and the picosecond laser with temporal jitter of less than 5 ps.
Q8. What is the driving force for the coherent amplitude Q?
The driving force F(x,t) for the coherent amplitude Q is0 009-2614/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)373proportional to the product of the laser and Stokes fields, F(x,t) a E=a.
Q9. What is the average power of the pulses from the UDR laser?
The pulses from this laser provide the exciting and probing light fields EL and EL*, respectively with an average power of z 10 mW.
Q10. How many times are the two modes separated by 189 cm-’?
Two modes separated by= 189 cm-’ are excited simultaneously, leading to the observed beat frequency of 5.7 THz.esting features are seen at early delay times.
Q11. What is the simplest way to explain the coherent signal?
(5)The authors assume T2, > T,,. From eq. (5) it is evident that the coherent signal consists of a sum of three contributions multiplied by the factor exp( -2tD Ty,‘) (the exponential decay can be removed from measured coherent data by multiplication with exp( 2tDTy,’ )).
Q12. What is the evolution of the coherent signal?
The coherently scattered light field consists of a sum over contributions originating from the differentvibrational modes j:2&S(tD)a cQJ(b> exp(-iwwtD+k$i) . (4) JThe evolution of the coherent signal is readily demonstrated for the case of two molecular transitions at frequencies w, and w,+Sw.
Q13. What is the corresponding frequency of the coherent signal?
one observes the scattered energy S, at the end of the sample, which is proportional to the time-integrated intensity, i.e. &s( tD) aJdt The authorEAs( t,tD) ) *.For sufficiently short exciting and probing pulses, t, < T2, and for late delay times, t+ tp, the salient features of the coherent signal are: (i) For a single vibrational mode the signal decays exponentially with exp( - 2tdT2).
Q14. What is the amplitude of the coherent Raman?
In a time-resolved coherent Raman experiment the coherent amplitude (q) which is generated at time zero is monitored by coherent Raman scattering of a delayed probing pulse EL2.
Q15. What is the polarization of the beams?
The anti-Stokes radiation EAs generated by the probing process passes the second polarizer and is detected after the broad-band spectrometer SP by the cooled photomultiplier PM.
Q16. How many ps is the decay of the coherent antiStokes signal?
3 the authors show the decay of the coherent antiStokes signal recorded from the binary mixture of cyclohexane and benzene of 1: 1 by volume.
Q17. What is the expected value of the vibrational excitation of an ensemble of molecules?
The vibrational excitation of an ensemble of molecules is described by the expectation value of the vibrational amplitude (q) = 4iQ exp( -io, t+ik, x) +c.c.
Q18. What is the amplitude of the coherent amplitude Qj?
The application of linear response theory to the excitation process leads to the following equation for the coherent amplitude Qj [ 1 ] :fQ, ew(i@,,) =K, s dt’ EL(f) B(f) --mxexp[(t’-t)lTz,] exp[ -iAw,t’] .
Q19. How many times did the excited force excite simultaneously?
The frequency of the exciting force was set to 900 cm- ’ in order to excite simultaneously Raman-active transitions in cyclohexane at 802 cm- ’ and in benzene at 992 cm-‘.
Q20. What is the amplitude of the Raman-active mode?
The total vibrational excitation may be described as the superposition of the individual states ( qj) with well defined amplitudes Q,, frequencies o, , and phase factors @,I:(4) =C<%) j=tiFQ, exp( - io,t+ik,x+i&) +c.c. (1)The coherent amplitude of a Raman-active mode with frequency oti can be excited via transient stimulated Raman scattering [ 91 by a pair of light pulses, the laser pulse EL, and the Stokes pulse Es.